In the physical common packet channels of the wide-band code division multiple access, a physical common packet channel is composed of an access prefix (AP), a conflict detection prefix (CD_P), a power control prefix (PCP), and the messages. The power transmitted by the access prefix is a climbing procedure: an initial access prefix is started to transmit by using an evaluated value of an open loop power, if the down-link acknowledgement information is not received, then the access prefix will be transmitted continuously by using the power bias of Δ P0. When the down-link acknowledgement information is received, then the conflict detection prefix will be transmitted by using the same power, after the acknowledgement information being received, the power control prefix and the messages will be transmitted by using the power bias of Δ Pp-m corresponding to the conflict detection prefix. An illustration diagram of the whole procedure is shown in FIG. 1.
Because the main function of the access prefix is used to determine an approximately suitable transmission power value, so the acquisition probability of which is not high under the power level of an access prefix which is the last one acquired successfully by the base station. However, the function of the conflict detection prefix is used to detect the conflict. Under the condition of a reasonable cost, the acquisition probability should be as high as possible. If the conflict detection prefixes are transmitted by the same power, then it will cause not only no conflict, but also no conflict detection prefix being able to be detected. Thus, this access procedure shall be restarted with a high cost. A method used presently is to increase the climbing steps of the access prefix, then the average power level and the acquisition probability of the access prefix, which is the last one acquired successfully, can be increased. The disadvantage thereof is the fluctuation of the power level (or the signal to noise ratio) of the access prefix, which is the last one accessed successfully, is large, so the object of the power control for making the access prefix climbing cannot be reached. However, the improvement of the acquisition performance of the conflict detection prefix is limited.
Power control prefixes subsequent to the conflict detection prefix are used to control the power, it can be seen from the above description, only an approximate power control can be obtained by using the climbing of the access prefix, this precision is not sufficient for demodulating the messages. Thus, the power control will be controlled by the power control prefixes to achieve a better power control result of the messages at the end of the power control prefix. The power control effect of the power control by using the power control prefix corresponds directly to the magnitude of the power of the power control prefix. If the power of the power control prefix is large, then the SIR evaluation of the base station will be more accurate, and the power control of the power control prefix will be more accurate. In contrast, the smaller the power of the power control prefix is, the more inaccurate the SIR evaluation of the base station is, and the worse the effect of the power control by using power control prefix will be. However, the power of the power control prefix cannot be increased limitlessly, because the increase of the power will cause the overhead and the interference with the other users. Thus, there is an optimum power bias value between the power of the power control prefix and the access prefix, which is the last one accessed successfully.
The demodulated power value of the subsequent messages is determined by the demodulation performance that the messages can reach, it is related to the frequency spread factor, encoding, and the channels of the messages, and the like. Normally, the value of which is different from the optimum power value of the power control prefix. Thus, there exists a power difference between the power control prefix and the control section of the messages. However, this power difference does not exist in the present physical common packet channels, thus, it is difficult to achieve the optimum comprehensive performance of the whole link.